Anneal-tunable Curie temperature and transport of La0.67Ca0.33MnO3

Resistivity measurements on a La0.67Ca0.33MnO3 film are reported for a series of argon anneals at successively higher temperatures. T-c, the ferromagnetic ordering temperature, increases uniformly with increasing annealing temperature and annealing time. Hence, T-c can be tuned by appropriate annealing. In order to fully anneal these samples, i.e., achieve bulk properties, it proves sufficient to anneal them in argon. Further annealing in oxygen produces only minor changes in the resistivity. Data from T-c up to 1200 K show activated conduction with rho = BTeEa/kT, the temperature dependence predicted by the Emin-Holstein theory of adiabatic polaron hopping. Their model fits both data from the partially annealed and fully annealed samples better than the variable range hopping or semiconductor models which have been used by previous workers. The activation energy E(a) and resistivity coefficient B decrease with increasing maximum anneal temperature. These changes, together with the increase in T-c, are consistent with an anneal induced relaxation of the Mn-O-Mn bond angle. The time dependent resistivity during annealing at a fixed temperature follows the equation rho=rho(0){1-D ln[1+(t-t(0))/tau]}, making it possible to acquire data in a reversible regime, and also to obtain an estimate of the annealing activation energy. (C) 1996 American Institute of Physics.